1. Field of the invention
The present invention relates to a method for manufacturing a high speed tool material for various tools. More specifically, the present invention relates to a high speed tool steel and a manufacturing method therefor, in which carbides are formed in the matrix in a uniform manner, thereby obtaining a high toughness and a high abrasion resistance.
2. Description of the prior art
Generally, a high speed tool steel is a high carbon alloy steel in which carbide forming elements are contained in large amounts. For example, one of them is W-Mo alloys, and others are W-Co alloys, Mo-Co alloys, and W-Mo-Co alloys.
If the high speed tool steel is to withstand against a high speed cutting operation, the abrasion resistance at a high temperature has to be superior, and the toughness has to be sufficient. Such mechanical properties of the high speed tool steel are decided by the size, shape, and distribution of the carbides within the alloy. The carbides in high speed tool steels are classified by containing metallic elements MC, M.sub.6 C, M.sub.2 C, M.sub.23 C.sub.6, and M.sub.7 C.sub.3. MC is a carbide containing vanadium as the major ingredient. M.sub.23 C.sub.6 is a carbide containing chrome as the major ingredient, and M.sub.6 C and M.sub.2 C are carbides containing tungsten and molybdenum as the major ingredients respectively. Specifically, if the mechanical properties such as abrasion resistance and toughness are to be superior, spherical carbides having a size of 2-3 .mu.m should be uniformly distributed.
Further, the high speed tool steels which have a manufacturing history of more than 100 years show that their mechanical properties are varied in accordance with the manufacturing methods.
The method for manufacturing the high speed tool steels is classified into an ordinary casting method and a powder metallurgical method. In a high speed tool steel billet which is manufactured by casting, coarse carbides are formed during the casting, and these coarse carbides are non-uniformly distributed within the billet, with the result that the workability becomes bad, and that the toughness and the shock resistance become low. Further, due to the growth of the coarse carbides and the severe segregation of the micro-structure, the kinds and contents of the alloy elements to be added are limited, this being a further disadvantage.
On the other hand, in the case where the high speed tool steel is manufactured by applying the powder metallurgical method, fine and uniform carbides can be obtained owing to the rapid cooling. Further, the amount of the alloy elements can be increased, and therefore, a material having a high abrasion resistance can be obtained.
For example, Japanese Patent Application Laid-open No. Sho-55-38961 discloses a method for manufacturing a high speed tool steel in which the powder metallurgical method is applied while restricting the content of tungsten. In this method, the growth of the M.sub.2 C carbide is inhibited, and instead, the growth of the MC and M.sub.6 C carbides are promoted, with the result that toughness and abrasion resistance are improved.
However, when manufacturing the high speed tool steel by applying the powder metallurgical method, there is required a complicated manufacturing process including the preparation of powder, a particle size sorting, a canning, a degassing treatment, a preform-making process, and a sintering process. Therefore, the control of the manufacturing conditions is difficult, and therefore, the manufacturing cost is increased.
Further, the M.sub.6 C carbides form carbide cells on the grain boundaries within the powder, and the carbide cells grow during the high temperature sintering so as to form continuous carbide cells. If these are to be destroyed, a high forging ratio is required. Further, in a coarse powder, there are generated the growth of coarse M.sub.6 C carbides and a segregation phenomenon, and therefore, toughness is adversely affected. Therefore, the control of particle size becomes difficult.
Meanwhile, the present applicant filed a patent application (under Korean Patent Application No. 94-38977) in which a method for manufacturing a high speed tool steel by applying a spray forming is disclosed unlike the casting and the powder metallurgical methods.
In this spray forming method, the MC+M.sub.2 C carbides are formed, and then, the M.sub.2 C carbides are made to be thermally decomposed. Then a hot forging is carried out. This spray forming method has many process advantages compared with the conventional casting and powder metallurgical methods. However, this spray forming method shows severe segregations, and therefore, it is applied to a particular composition, while it has not been commercialized.